1. Field of the Invention
This invention relates to electronic tone synthesis and in particular is concerned with a means for generating tones with time varying harmonic strengths.
2. Description of the Prior Art
An elusive goal in the design of electronic musical instruments is the ability to realistically imitate the sounds of conventional acoustic type orchestral musical instruments. The best results have been obtained for electronic musical instruments which imitate wind-blown pipe organs and harpsichords. The principal reason for obtaining good imitative results for these instruments is that they are essentially mechanical tone generators. The tone generation mechanism is automatic in operation and the musician needs only to actuate on-off switches.
It has long been recognized that with the notable exception of conventional organ tones, almost all tones produced by musical instruments exhibit tone spectra which are time variant in composition. In recent years there has been considerable research devoted to the detailed nature of the sounds produced by acoustic orchestral musical instruments. The application of large digital computers to this research has yielded large amounts of spectral data. It has generally been determined that the various harmonics, which constitute the tonal spectrum of a musical instrument, vary with time from the onset until the cessation of tone production. The individual harmonics have time variant strength patterns which are essentially independent of each other.
Musical tone synthesizers have been implemented which use stored tabular values of the individual harmonic time variations to synthesis tones. Frequently the synthesized tones cannot easily be distinguished from those produced by the parent musical instrument.
The large amount of data that must be stored to individually control the time variant behavior of a set of harmonics is a serious detriment to a practical implementation of a musical tone synthesizer which creates tones by computing a Fourier transform using the set of harmonics. Attempts have been made to reduce the quantity of stored data by resorting to the use of a series of straight line segments to approximate the amplitude-time function curves for each member of the set of harmonics that characterize a selected musical tone. The practical problem that must be solved in a linear approximation process is to choose the smallest number of individual line segments for the approximation to a curve such that a "realistic" tone is produced while reducing the amount of data for the line segments that must be stored for each of the harmonics used to synthesize the musical tone.
A survey of various algorithms for approximating curves with linear piecewise segments is contained in the technical article: Strown, J., "Approximation and Sytatic Analysis of Amplitude and Frequency Functions for Digital Sound Synthesis." Computer Music Journal, Vol. 4, No. 3 (1980) pp 3-24.
While the use of piecewise linear segments to approximate a function is a widely used technique, it is by no means the only available technique nor is it even the best method to use in any arbitrary situation. The choice of an approximating function is influenced by the allowable approximation error and the practical problem arising from generating the selected approximating function from a minimal data set of curve generating parameters. For a linear line segment, the approximation is determined by specifying the slope of the line and the starting point. The total number of required line segments can only be determined by examination of the given function which represents the time variation of a harmonic component of the musical tone. Each line segment requires a data set comprising the line slope, starting point, and time at which the line is to be used in the function approximation.
If the approximate shape of the harmonic time function is known, and is not completely arbitrary, then frequently the amount of stored data for the approximation can be reduced in comparison to that required for a piecewise linear approximation. Such a curve approximation system is described in U.S. Pat. No. 4,211,138 entitled "Harmonic Formant Filter For An Electronic Musical Instrument." The patent describes a curve synthesis technique in which a tone formant filter is constructed by adding a number of standard resonant curves together after displacing the resonant frequencies of the individual component resonant curves.